US20130076325A1 - Voltage regulator - Google Patents
Voltage regulator Download PDFInfo
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- US20130076325A1 US20130076325A1 US13/612,754 US201213612754A US2013076325A1 US 20130076325 A1 US20130076325 A1 US 20130076325A1 US 201213612754 A US201213612754 A US 201213612754A US 2013076325 A1 US2013076325 A1 US 2013076325A1
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- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
- G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
- G05F1/10—Regulating voltage or current
- G05F1/46—Regulating voltage or current wherein the variable actually regulated by the final control device is dc
- G05F1/56—Regulating voltage or current wherein the variable actually regulated by the final control device is dc using semiconductor devices in series with the load as final control devices
- G05F1/575—Regulating voltage or current wherein the variable actually regulated by the final control device is dc using semiconductor devices in series with the load as final control devices characterised by the feedback circuit
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- the disclosure relates to a low dropout (LDO) regulator, and more particularly to a LDO regulator with high stability.
- LDO low dropout
- Voltage regulators are commonly used in the power management systems of computers, mobile phones, automobiles and many other electronic products. Generally, voltage regulators are configured to convert unstable power supply voltage into stable power supply voltage.
- a low dropout (LDO) regulator has a low input-to-output voltage difference between an input terminal where an unstable power supply voltage is inputted and an output terminal where a stable power supply voltage is outputted.
- Dropout voltage refers to the input-to-output voltage difference, whereby the regulator ceases to regulate against further reductions in the input voltage. Ideally, the dropout voltage should be as low as possible, to reduce the power consumption while still maintaining regulation performance.
- An embodiment of a voltage regulator comprises a pass transistor, an operational amplifier, and a voltage divider circuit.
- the pass transistor receives a supply voltage to generate a regulated output voltage according to a control signal.
- the operational amplifier generates the control signal according to a feedback voltage.
- the voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage.
- the voltage divider circuit comprises a string of resistors and a stabilization element.
- the string of resistors is coupled to the pass transistor and comprises a plurality of resistors.
- the stabilization element is coupled to the string of resistors and receives the regulated output voltage.
- a voltage regulator comprises a first transistor, an operational amplifier, and a voltage divider circuit.
- the first transistor receives a supply voltage to generate a regulated output voltage at an output node according to a control signal.
- the operational amplifier generates the control signal according to a difference between a reference voltage and a feedback voltage.
- the voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage.
- the voltage divider circuit comprises a string of resistors and a stabilization element.
- the string of resistors is coupled to the first transistor and comprises a plurality of resistors.
- the second transistor is coupled to the resistors and comprises a gate coupled to the output node.
- a voltage regulator comprises a pass transistor and a voltage divider circuit.
- the pass transistor receives a supply voltage to generate a regulated output voltage according to a control signal.
- the control signal is generated according to a feedback voltage.
- the voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage.
- the voltage divider circuit comprises a string of resistors and a stabilization element.
- the string of resistors is coupled to the pass transistor and comprising a plurality of resistors.
- the resistors and a plurality of parasitic capacitance generate a pole in a low frequency region at the feedback node.
- the stabilization element is coupled to the resistors and pushes the pole to a high frequency region.
- FIG. 1 shows a block diagram of a voltage regulator according to one embodiment of the invention
- FIG. 2 shows a circuit diagram of a voltage regulator according to one embodiment of the invention
- FIG. 3 shows a partial circuit diagram at an input nodes of an operational amplifier of the voltage regulator of FIG. 2 ;
- FIG. 4 is a schematic diagram showing alternative current (AC) signal analysis results of the voltage regulator of FIG. 2 .
- FIG. 1 shows a block diagram of a voltage regulator 100 according to an embodiment of the invention.
- the voltage regulator 100 may be a low dropout (LDO) regulator in this embodiment, and may comprise a pass transistor 101 , an operational amplifier 102 , and a voltage divider circuit 103 .
- the pass transistor 101 receives an unregulated supply voltage V IN and generates a regulated output voltage V OUT according to a control signal Ctrl.
- the voltage divider circuit 103 provides a feedback voltage at a feedback node FB according to the regulated output voltage V OUT .
- the operational amplifier 102 is coupled to the feedback node FB and generates the control signal Ctrl according to the feedback voltage.
- the voltage divider circuit 103 comprises a string of resistors 131 and a stabilization element 132 .
- the string of resistors 131 comprises a plurality of resistors (not labeled).
- the stabilization element 132 is coupled to the resistors and comprises a control node (not shown) receiving the regulated output voltage V OUT for stabilizing operations of the voltage regulator 100 .
- the stabilization element 132 generates a high frequency pole, with a frequency much higher than the operation frequency band of the voltage regulator, at the feedback node FB.
- the high frequency pole is generated by the stabilization element 132 by pushing a pole, which would cause the system to operate unstably in a conventional voltage regulator, to a high frequency region, so as to maintain the stability of the voltage regulator 100 .
- FIG. 2 shows a circuit diagram of a voltage regulator 200 according to one embodiment, e.g. the embodiment shown as FIG. 1 , of the invention.
- the voltage regulator 200 may be a low dropout (LDO) regulator, and may comprise a pass transistor 201 , an operational amplifier 202 , and a voltage divider circuit 203 .
- the pass transistor 201 comprises a gate 201 a coupled to the operational amplifier 202 for receiving the control signal Ctrl, and regulates the unregulated supply voltage V according to the control signal Ctrl, thereby generating the regulated output voltage V OUT at the output node (not labeled).
- the operational amplifier 202 comprises two input nodes 202 a and 202 b for respectively receiving the reference voltage V REF and the feedback voltage V FB , and generates the control signal Ctrl according to a difference between the reference voltage V REF and the feedback voltage V FB .
- the voltage divider circuit 203 comprises a string of resistors 203 a and a stabilization element 203 b.
- the string of resistors 230 a at least comprises resistors R 1 and R 2 .
- the resistor R 1 is coupled between the pass transistor 201 and the stabilization element 203 b, the stabilization element 203 b is coupled between the resistor R 1 and the feedback node FB, and the resistor R 2 is coupled between the feedback node FB and a ground node 204 .
- the stabilization element 203 b may include, e.g. the transistor 232 shown in FIG. 2 .
- the transistor 232 may be an N-type metal oxide semiconductor (NMOS) transistor. Note that because a gate (i.e. a control node) of the transistor 232 is coupled to the output node for receiving the regulated output voltage V OUT , a gate voltage of the transistor 232 is increased to be higher than a drain voltage of the transistor 232 . Because the gate-drain voltage difference is greater than the threshold voltage, the transistor 232 operates in a linear region.
- NMOS N-type metal oxide semiconductor
- FIG. 3 shows a partial circuit diagram for the input nodes 202 a and 202 b of the operational amplifier 202 according to the voltage regulator 200 of FIG. 2 .
- the input nodes 202 a and 202 b of the operational amplifier 202 may comprise a differential MOS pair 205 and 206 and a plurality of parasitic capacitance, such as the parasitic capacitance Cgs and Cgd parasitized at the input nodes 202 a and 202 b of the operational amplifier 202 as shown in the figure.
- the resistors in the voltage divider circuit 203 are usually selected to have large resistance, such as several Mega-ohm.
- one input node e.g.
- the operational amplifier 202 is coupled to the voltage divider circuit 203 at the feedback node FB, if there is no stabilization element 203 b coupled to the feedback node FB, a pole in a low frequency region (low frequency pole) would be created at the feedback node FB by the mutually coupled parasitic capacitance and the resistors, wherein the frequency of the low frequency pole would be:
- the R 1 represents the resistance of the resistor R 1
- the R 2 represents the resistance of the resistor R 2
- the C gs represents the capacitance of the capacitor Cgs
- the C gd represents the capacitance of the capacitor Cgd.
- the frequency of the pole as derived from Eq. 1 would be 300 KHz. Because an operation frequency band of a voltage regulator is generally distributed from 200 KHz to 500 KHz, the low frequency pole would seriously affect the stability of the voltage regulator 200 if there is no stabilization element 203 b.
- the transistor 232 is coupled at the feedback node FB so as to stabilize the operations of the voltage regulator 200 .
- the turn-on resistance r ON of the transistor 232 is very small. Therefore, the transistor 232 may be regarded as a small resistor for direct current (DC) and barely affect the DC component in the regulated output voltage V OUT .
- the transistor 232 may further reduce the resistance at the feedback node FB when looking upward from the feedback node FB, thereby pushing the pole (that is, the above-mentioned low frequency pole), created by the parasitic capacitance Cgs and Cgd and at the feedback node FB, from the low frequency region to the high frequency region.
- the pole that is, the above-mentioned low frequency pole
- FIG. 4 is a schematic diagram showing the AC signal analysis results according to the embodiment of FIG. 2 .
- the AC component in the control signal at the gate of the pass transistor 201 is (A ⁇ V i ), where A represents a gain of the operational amplifier 202 .
- the AC component in the regulated output voltage V OUT is ( ⁇ V i ⁇ A ⁇ gm ⁇ r out ), where r out represents the resistance looking from the output node into the voltage regulator 200 and gm represents the transconductance of the pass transistor 201 .
- the drain-source current of the transistor 232 may be:
- i ds ⁇ ⁇ ⁇ C ox ⁇ W L ⁇ ( - V i ⁇ A ⁇ gm ⁇ r out - V i - V th ) ⁇ ⁇ - ⁇ ⁇ C ox ⁇ W L ⁇ ( V i ⁇ A ⁇ gm ⁇ r out ) Eq . ⁇ 2
- V th is the threshold voltage of the transistor 232
- ⁇ is the charge carrier effective mobility
- C ox is the unit capacitance of the gate oxide
- W is the gate width of the transistor 232
- L is the gate length of the transistor 232 .
- the input impedance of the AC voltage V i may further be derived from Eq. 2 as:
- r in is the input impedance of the transistor 232 when looking upward from the feedback node FB. Because the gain A and the transconductance gm are generally very large, the input impedance r in is very small as shown in Eq. 3, when the transistor 232 is coupled to the feedback node FB, the frequency of the low frequency pole created at the feedback node FB becomes:
- the stabilization element 203 b may also comprise more than one transistor.
- the stability and the ability to resist process variation may further be improved.
- the gate of the transistor 232 may not have to be directly connected to the output node V OUT as shown in FIG. 2 and FIG. 4 .
- an electrostatic discharge protection circuit may be coupled to the control node (i.e. between the gate of the transistor 232 and the output node V OUT ) of the stabilization element 132 .
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Abstract
Description
- This Application claims priority of China Patent Application No. 201110297992.5, filed on Sep. 27, 2011, the entirety of which is incorporated by reference herein.
- 1. Field of the Invention
- The disclosure relates to a low dropout (LDO) regulator, and more particularly to a LDO regulator with high stability.
- 2. Description of the Related Art
- Voltage regulators are commonly used in the power management systems of computers, mobile phones, automobiles and many other electronic products. Generally, voltage regulators are configured to convert unstable power supply voltage into stable power supply voltage. A low dropout (LDO) regulator has a low input-to-output voltage difference between an input terminal where an unstable power supply voltage is inputted and an output terminal where a stable power supply voltage is outputted. “Dropout voltage” refers to the input-to-output voltage difference, whereby the regulator ceases to regulate against further reductions in the input voltage. Ideally, the dropout voltage should be as low as possible, to reduce the power consumption while still maintaining regulation performance.
- In the conventional LDO regulator design, a low frequency pole, at about 200 KHz to 500 KHz, is usually generated at the feedback terminal Because the low frequency pole falls within the operation frequency band of the LDO regulator, the stability of the LDO regulator is seriously downgraded. However, stability is an important factor of the LDO regulator.
- Therefore, a novel LDO regulator, which can push the pole to a high frequency band while still maintaining high stability, is highly required.
- Voltage regulators are provided. An embodiment of a voltage regulator comprises a pass transistor, an operational amplifier, and a voltage divider circuit. The pass transistor receives a supply voltage to generate a regulated output voltage according to a control signal. The operational amplifier generates the control signal according to a feedback voltage. The voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage. The voltage divider circuit comprises a string of resistors and a stabilization element. The string of resistors is coupled to the pass transistor and comprises a plurality of resistors. The stabilization element is coupled to the string of resistors and receives the regulated output voltage.
- Another embodiment of a voltage regulator comprises a first transistor, an operational amplifier, and a voltage divider circuit. The first transistor receives a supply voltage to generate a regulated output voltage at an output node according to a control signal. The operational amplifier generates the control signal according to a difference between a reference voltage and a feedback voltage. The voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage. The voltage divider circuit comprises a string of resistors and a stabilization element. The string of resistors is coupled to the first transistor and comprises a plurality of resistors. The second transistor is coupled to the resistors and comprises a gate coupled to the output node.
- Another embodiment of a voltage regulator comprises a pass transistor and a voltage divider circuit. The pass transistor receives a supply voltage to generate a regulated output voltage according to a control signal. The control signal is generated according to a feedback voltage. The voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage. The voltage divider circuit comprises a string of resistors and a stabilization element. The string of resistors is coupled to the pass transistor and comprising a plurality of resistors. The resistors and a plurality of parasitic capacitance generate a pole in a low frequency region at the feedback node. The stabilization element is coupled to the resistors and pushes the pole to a high frequency region.
- A detailed description is given in the following embodiments with reference to the accompanying drawings.
- The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
-
FIG. 1 shows a block diagram of a voltage regulator according to one embodiment of the invention; -
FIG. 2 shows a circuit diagram of a voltage regulator according to one embodiment of the invention; -
FIG. 3 shows a partial circuit diagram at an input nodes of an operational amplifier of the voltage regulator ofFIG. 2 ; and -
FIG. 4 is a schematic diagram showing alternative current (AC) signal analysis results of the voltage regulator ofFIG. 2 . - The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
-
FIG. 1 shows a block diagram of avoltage regulator 100 according to an embodiment of the invention. Thevoltage regulator 100 may be a low dropout (LDO) regulator in this embodiment, and may comprise apass transistor 101, an operational amplifier 102, and avoltage divider circuit 103. Thepass transistor 101 receives an unregulated supply voltage VIN and generates a regulated output voltage VOUT according to a control signal Ctrl. Thevoltage divider circuit 103 provides a feedback voltage at a feedback node FB according to the regulated output voltage VOUT. The operational amplifier 102 is coupled to the feedback node FB and generates the control signal Ctrl according to the feedback voltage. - According to one embodiment of the invention, the
voltage divider circuit 103 comprises a string ofresistors 131 and astabilization element 132. The string ofresistors 131 comprises a plurality of resistors (not labeled). Thestabilization element 132 is coupled to the resistors and comprises a control node (not shown) receiving the regulated output voltage VOUT for stabilizing operations of thevoltage regulator 100. To be more specific, thestabilization element 132 generates a high frequency pole, with a frequency much higher than the operation frequency band of the voltage regulator, at the feedback node FB. The high frequency pole is generated by thestabilization element 132 by pushing a pole, which would cause the system to operate unstably in a conventional voltage regulator, to a high frequency region, so as to maintain the stability of thevoltage regulator 100. -
FIG. 2 shows a circuit diagram of avoltage regulator 200 according to one embodiment, e.g. the embodiment shown asFIG. 1 , of the invention. Thevoltage regulator 200 may be a low dropout (LDO) regulator, and may comprise apass transistor 201, anoperational amplifier 202, and avoltage divider circuit 203. Thepass transistor 201 comprises agate 201 a coupled to theoperational amplifier 202 for receiving the control signal Ctrl, and regulates the unregulated supply voltage V according to the control signal Ctrl, thereby generating the regulated output voltage VOUT at the output node (not labeled). - The
operational amplifier 202 comprises twoinput nodes voltage divider circuit 203 comprises a string of resistors 203 a and astabilization element 203 b. The string of resistors 230 a at least comprises resistors R1 and R2. The resistor R1 is coupled between thepass transistor 201 and thestabilization element 203 b, thestabilization element 203 b is coupled between the resistor R1 and the feedback node FB, and the resistor R2 is coupled between the feedback node FB and aground node 204. - According to an embodiment of the invention, the
stabilization element 203 b may include, e.g. thetransistor 232 shown inFIG. 2 . Thetransistor 232 may be an N-type metal oxide semiconductor (NMOS) transistor. Note that because a gate (i.e. a control node) of thetransistor 232 is coupled to the output node for receiving the regulated output voltage VOUT, a gate voltage of thetransistor 232 is increased to be higher than a drain voltage of thetransistor 232. Because the gate-drain voltage difference is greater than the threshold voltage, thetransistor 232 operates in a linear region. -
FIG. 3 shows a partial circuit diagram for theinput nodes operational amplifier 202 according to thevoltage regulator 200 ofFIG. 2 . Theinput nodes operational amplifier 202 may comprise adifferential MOS pair input nodes operational amplifier 202 as shown in the figure. In general, in order to reduce the quiescent current, the resistors in thevoltage divider circuit 203 are usually selected to have large resistance, such as several Mega-ohm. However, as shown inFIG. 2 , because one input node (e.g. 202 b) of theoperational amplifier 202 is coupled to thevoltage divider circuit 203 at the feedback node FB, if there is nostabilization element 203 b coupled to the feedback node FB, a pole in a low frequency region (low frequency pole) would be created at the feedback node FB by the mutually coupled parasitic capacitance and the resistors, wherein the frequency of the low frequency pole would be: -
- where the R1 represents the resistance of the resistor R1, the R2 represents the resistance of the resistor R2, the Cgs represents the capacitance of the capacitor Cgs, and the Cgd represents the capacitance of the capacitor Cgd.
- Suppose that R1=R2=1MΩ and Cgs=Cgd=500 fF, the frequency of the pole as derived from Eq. 1 would be 300 KHz. Because an operation frequency band of a voltage regulator is generally distributed from 200 KHz to 500 KHz, the low frequency pole would seriously affect the stability of the
voltage regulator 200 if there is nostabilization element 203 b. - Therefore, in one embodiment of the invention, the
transistor 232 is coupled at the feedback node FB so as to stabilize the operations of thevoltage regulator 200. As previously described, because thetransistor 232 operates in the linear region , the turn-on resistance rON of thetransistor 232 is very small. Therefore, thetransistor 232 may be regarded as a small resistor for direct current (DC) and barely affect the DC component in the regulated output voltage VOUT. In another perspective, regarding the alternative current (AC) component, thetransistor 232 may further reduce the resistance at the feedback node FB when looking upward from the feedback node FB, thereby pushing the pole (that is, the above-mentioned low frequency pole), created by the parasitic capacitance Cgs and Cgd and at the feedback node FB, from the low frequency region to the high frequency region. -
FIG. 4 is a schematic diagram showing the AC signal analysis results according to the embodiment ofFIG. 2 . As shown inFIG. 4 , when an AC voltage Vi is connected to the resistor R2, the AC component in the control signal at the gate of thepass transistor 201 is (A×Vi), where A represents a gain of theoperational amplifier 202. The AC component in the regulated output voltage VOUT is (−V i×A×gm×rout), where rout represents the resistance looking from the output node into thevoltage regulator 200 and gm represents the transconductance of thepass transistor 201. In addition, a gate-source voltage of thetransistor 232 is Vgs=[(−Vi×A×gm×rout)−Vi]. - Based on the values derived above, the drain-source current of the
transistor 232 may be: -
- where Vth is the threshold voltage of the
transistor 232, μ is the charge carrier effective mobility, Cox is the unit capacitance of the gate oxide, W is the gate width of thetransistor 232 and L is the gate length of thetransistor 232. - The input impedance of the AC voltage Vi may further be derived from Eq. 2 as:
-
- where rin is the input impedance of the
transistor 232 when looking upward from the feedback node FB. Because the gain A and the transconductance gm are generally very large, the input impedance rin is very small as shown in Eq. 3, when thetransistor 232 is coupled to the feedback node FB, the frequency of the low frequency pole created at the feedback node FB becomes: -
- As shown in Eq. 4, because the input impedance rin is very small, the low frequency pole created at the feedback node FB will be pushed to a high frequency region and becomes a high frequency pole. Since the frequency of the high frequency pole is much higher than the operation frequency band (as described above, usually in several KHz) of the
voltage regulator 200, the high frequency pole will not affect the stability of thevoltage regulator 200. In addition, because the circuit area required for a transistor is small, the increased circuit area due to the addition of the transistors, as thestabilization element 203 b to thevoltage regulator 200, is small. Note that in another embodiment of the invention, thestabilization element 203 b may also comprise more than one transistor. By taking the programmable advantages of the transistors, the stability and the ability to resist process variation may further be improved. In addition, the gate of thetransistor 232 may not have to be directly connected to the output node VOUT as shown inFIG. 2 andFIG. 4 . For example, an electrostatic discharge protection circuit may be coupled to the control node (i.e. between the gate of thetransistor 232 and the output node VOUT) of thestabilization element 132. - While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.
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CN201110297992 | 2011-09-27 | ||
CN2011102979925A CN103019288A (en) | 2011-09-27 | 2011-09-27 | Voltage regulator |
CN201110297992.5 | 2011-09-27 |
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US11467613B2 (en) * | 2020-07-15 | 2022-10-11 | Semiconductor Components Industries, Llc | Adaptable low dropout (LDO) voltage regulator and method therefor |
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Cited By (3)
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WO2014191787A1 (en) * | 2013-05-29 | 2014-12-04 | Freescale Semiconductor, Inc. | Voltage regulator, application-specific integrated circuit and method for providing a load with a regulated voltage |
US9841777B2 (en) | 2013-05-29 | 2017-12-12 | Nxp Usa, Inc. | Voltage regulator, application-specific integrated circuit and method for providing a load with a regulated voltage |
US11467613B2 (en) * | 2020-07-15 | 2022-10-11 | Semiconductor Components Industries, Llc | Adaptable low dropout (LDO) voltage regulator and method therefor |
Also Published As
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CN103019288A (en) | 2013-04-03 |
US8810218B2 (en) | 2014-08-19 |
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